Module administration

[bnjbvr]

It'd be nice to be able to set some secrets for the bot modules, to enable new use cases:

• posting to Twitter/Mastodon would require API tokens
• expanding Gitlab/Github links would need a configuration per Matrix room, likely

One could imagine that the administration could also be public, but we really want it to be accessible only to one or a subset of admins, so not everybody can show up in a room and wreak havoc indirectly through the bot.

I think a first good step would be to have an admin user id (that could be set via the environment variables), and only them could interact with the administration command. Since this command would need to be priviledged (as in: might read admin metadata from command modules), it would likely need to be implemented as a host command, and not as a wasm module.

Likely requires #4.